Electrolysis of liquids is almost always performed between at least two electrodes, and in most cases, a separator physically separates the electrodes. To enable electric contact between the electrodes, an electrolyte is employed. Many electrolytic cells known in the art are devices in which the electrolyte and the separator are two distinct functional elements. For example, in a NaCl electrolytic cell, the separator is frequently an asbestos diaphragm or a mercury intermediate electrode, and a highly alkaline NaCl solution serves as electrolyte. However, there are several disadvantages to such a configuration. One disadvantage is that asbestos and mercury pose a severe health hazard. Another disadvantage is that the electrolyte tends to corrode the electrolysis container, often resulting in leakage. A still further disadvantage is that liquid electrolytes usually have a limited life span due to contamination.
To overcome at least some of the problems associated with physically separated electrolyte and separator, polymer electrolytes were developed that combine the electrolyte and the separator into one physical entity. The conductivity of such polymer electrolytes is generally achieved by introduction of ion exchange groups into a polymeric structure. When only one of a cationic and anionic ion exchange group is employed, single ions can migrate through the separator. When both cationic and anionic ion exchange groups are built into the polymer, ion pairs can migrate though the polymer. Polymer electrolytes are frequently superior to a combination of separator and electrolyte, because polymer electrolytes allow a denser packaging of an electrolytic cell. Furthermore, since there is usually no liquid electrolyte in electrolytic cells with polymer electrolytes, problems with limited life span due to contamination of the electrolyte are averted. Moreover, polymer electrolytes often circumvent the problems that usually arise from corrosion due to a fluid electrolyte.
Various polymer electrolytes are known in the art, including poly(ethylene oxide)- and poly(propylene oxide) based compounds, or polysulfone- and polyvinylidene compounds. Such compounds are relatively inexpensive and can be utilized in various applications. Unfortunately, some of these polymer electrolytes suffer from relatively low ion conductivity. Other polymer electrolytes have only limited chemical stability making them less useful for longer term applications.
Despite many advantages, almost all of the known polymer electrolytes suffer from a general drawback in that they are stable only at relatively low temperatures. Temperature stability of polymer electrolytes, however, is relatively important when cost-effectiveness of the electrolytic process is desired. In general, higher temperatures typically increase the rate of most thermodynamically and/or kinetically controlled reactions, including many electrochemical reactions. For example, the efficiency of water electrolysis benefits from an increase in temperature due to a decrease in the thermodynamic potential and a decrease in electrode polarization. Besides the thermodynamic and electrochemical effects of higher temperatures, further advantageous effects may occur. For example, temperatures in the range of approximately 150.degree. C. to 250.degree. C. tend to promote a superior carbon monoxide tolerance during electrolysis of water. Still more advantageously, a temperature range of about 150.degree. C. to 250.degree. C. allows direct oxidation of substrates other than water, including ethanol, other alcohols and hydrocarbons.
Recently, polymer electrolytes with somewhat improved thermal stability have been synthesized and are known in the art. For example, perfluorinated hydrocarbon sulfonate ionomers, such as Nafion.TM. (a perfluorinated hydrocarbon with sulfonic acid groups), are now commercially available. However, despite their enhanced thermal stability many difficulties still persist. One problem is that perfluorinated hydrocarbon sulfonate ionomers are relatively expensive. Another problem is that such ionomers tend to decompose at temperatures of about 80.degree. C. and above when they are used over a prolonged period of time.
To achieve higher thermal stability of polymer electrolytes, various approaches have been pursued. One approach is to utilize compounds with known higher thermostability. For example, U.S. Pat. No. 5,548,055 to Narang et al., demonstrate polymer electrolytes based on polysiloxanes. However, polysiloxane based polymer electrolytes typically need admixing with plasticizers to enhance ionic conductivity, and further combination with other materials such as polyvinylidene fluoride to improve mechanical strength. In another approach, U.S. Pat. No. 5,741,408 to Helmer-Metzmann, the author shows that cross-linking polymeric strands in a polymer electrolyte can improve the stability of a high temperature polymer electrolyte. However, crosslinking generally involves at least one additional step in the preparation of the final polymer electrolyte. More disadvantageously, crosslinking reactions usually require a plurality of reactive groups in the polymer. When the crosslinking reaction is not forced entirely to completion, remaining unreacted crosslinking groups may render the polymer more susceptible to chemical instability. In a still further approach, Ogata et al. describe in U.S. Pat. No. 5,403,675 a sulfonated polyphenylene polymer electrolyte. However, polyphenylenes are generally difficult to dissolve, and solubilizing side groups such as alcohol-, aldehyde- or alkaryl groups are frequently introduced. Solubilizing side groups, however, may introduce a chemical instability under high temperature conditions.
Novel polymer electrolytes with increased thermal stability have been developed, however, known polymer electrolytes still suffer from several disadvantages. Therefore, there is still a continuing need for improved high temperature polymer electrolytes.